1. Field of the Invention
This invention relates to an image formation apparatus such as a copying apparatus or the like in which control is effected using an A/D converter for converting an analog signal into a digital signal.
2. Description of the Prior Art
In recent years, a great quantity of microcomputers have been produced inexpensively and applied to various types of automatic control apparatus, and apparatus using an A/D converter have been proposed.
For example, in an electrophotographic apparatus wherein the surface potential of a photosensitive medium is measured and the outputs of a primary and a secondary charger are varied using a microcomputer to thereby stabilize the potential, normal image reproduction cannot be expected if the value resulting from A/D conversion of the measured surface potential is abnormal.
Also, in an automatic control apparatus for a motor wherein the field current or the number of revolutions of the motor is controlled to a predetermined or desired value by a microcomputer through the use of an A/D converter, the motor may be stopped or recklessly run due to an abnormality of the A/D converter.
The abnormal operation of the A/D converter includes a malfunction resulting either from damage caused by an overvoltage or the like or from deterioration occurring with time.
To prevent the inconvenience or hazard in image formation resulting from such abnormal operation, it is necessary to remove from a base plate an IC chip forming the A/D converter and determine a fault by a checker before a desired apparatus operation is started.
However, effecting such check-up before the operation is started requires much labor and time and moreover, involves the necessity of preparing an IC checker.
In an electrophotographic image formation apparatus, what particularly affects the electrostatic latent image potential of the photosensitive medium is the corona discharge for applying charge and discharge to the photosensitive medium. Description will hereinafter be made by taking as an example a primary charger for pre-charging and a secondary charger for applying discharge simultaneously with application of light. In this case, it is practised to measure and operate the electrostatic latent image potential of the photosensitive medium, vary the outputs of the primary and secondary chargers and control the electrostatic latent image potential to a proper value. However, the circuit for carrying out such control is complicated and offers a problem that it is liable to become defective and moreover the defectiveness is difficult to discover. This in turn leads to much time required for the maintenance and check of the apparatus and accordingly to poor serviceability.
To improve this, it would occur to mind to use a microcomputer in the operational circuit. However, simple use of a microcomputer would not enable discovery of the defectiveness.